1. Field of the Invention
The present invention relates to the storage of digital data and more particularly to high density storage of digital data using a capacitive electronic disc (CED).
2. State of the Art
In the early 1980's, RCA commercialized a relatively low-cost video disc system using capacitance pick up technology to detect video and audio signals placed in grooves on a capacitive electronic disc (CED). Although the CED bears a superficial similarity to a vinyl LP record in that the signal information is placed in grooves, striking differences distinguish the CED from an audio disk. For example, whereas a typical audio disk has a groove width of approximately 0.004 inches, a CED has a groove width of approximately 0.0001 inches such that 40 video disc grooves may be accommodated in the same space as a single audio disk groove. A relatively simple stylus contacts the video disc surface and tracks in the grooves, picking up capacitance variations associated with sub-micron features in the grooves. More particularly, video and audio carriers are FM-modulated and placed on the disc by varying the depth of the groove with the carrier signals. Because of the very small size of the groove (only 2.5 .mu.m wide), the modulation of the groove depth is also necessarily very small, approximately 850 .ANG. peak-to-peak maximum for video. In FIG. 1,, modulation of the groove depth of a CED 12 is represented by dashed lines. FIG. 1 represents a radial cross section of the CED. A stylus 10 is shown tracking in a groove in contact with the disc surface.
The length of the stylus 10 is several times greater than the longest recorded wavelength on the video disc. Therefore, as the stylus travels over the modulation in the groove, its vertical position remains constant. A thin metalized electrode is placed on the trailing surface of the stylus and acts as one plate of a "capacitor" formed by the electrode and the surface of the video disc. The video disc is made of a conductive plastic and acts as the other plate of the capacitor. As the disc rotates,, the distance between the bottom edge of the stylus electrode and the modulation in the groove varies as a function of the modulation. The distance between the plates of the capacitor therefore varies at the frequency of the modulation, changing the capacitance between the stylus and the disc.
The video signal recorded on the disc is a frequency-modulated 5 MHz video carrier. A black level of the video signal is represented by a frequency of 5 MHz, and a white level is represented by deviation of the video carrier signal to 6.3 MHz. Furthermore, synchronization signals cause the video carrier frequency to deviate to 4.3 MHz.
In the RCA VideoDisc system, the vertical blanking intervals of each field are aligned radially on the disc. Recorded on the disc during each vertical blanking interval is a DAXI (digital auxiliary information) code used to uniquely identify each field in a video program. Using "stylus kicker" coils, the stylus may be caused to skip between grooves of the video disc to provide rapid access and visual search modes of operation.
Despite the VideoDisc system's convenience of operation and technical superiority in terms of signal quality, the introduction of VCRs featuring full-function recording by the user prevented the Video disc system from achieving commercial success. Production of the RCA VideoDisc system continued for only a few years.
The RCA VideoDisc recording/playback signaling channel has been thoroughly studied as detailed in the following references:
1) "Capacitive Pickup and the Buried Subcarrier Encoding System for the RCA VideoDisc", J. K. Clemens, RCA Review, Mar. 1978, Vol. 39, No. 1, p. 33. PA0 2) "The Influence of Carrier-to-Noise Ratio and Stylus Life on the RCA VideoDisc System Parameters", M. D. Ross, J. K. Clemens, and R. C. Palmer, RCA Review, Sep. 1981, Vol. 42, No. 3, p. 394. PA0 3) "An Analysis of the Signal and Noise Transfer Properties of Capacitive Styli for the RCA VideoDisc System", J. J. Gibson, Technical Report PRRL-85-TR-017, David Sarnoff Research Center.
The channel can be described as signal plus white Gaussian noise, but with non-linearities. The signal is nonlinear in amplitude and has a linear roll off at shorter wavelengths, which is accentuated when the stylus is lifted off the surface. In addition, there is a small fixed phase shift that can vary under stylus displacement. There is also a small amount of multiplicative noise. Since the additive white Gaussian noise dominates the perturbations of the signal, however, the signal system must be chosen correctly to optimize the final signal-to-noise ratio in the presence of the non-linearities. An angle modulation system, FM, was chosen and optimized for the VideoDisc system.
There will always be a market demand for increasing the volume of random access information available in the portable or mobile computing environment, or as part of the solution to high density packaging constraints in stationary equipment. The present invention enables realization of digital data density that is superior to that available today, and offers the potential to advance beyond the theoretical limit of other systems.
The present invention capitalizes on existing technology in analog capacitive video disc recording (CED) by developing new techniques to provide digital data storage having theoretical storage density that is unsurpassed by all other disk media including optical and magnetic technology. The capacitive disc technology provides a combination of benefits including high density data storage and rapid access.